It could be a wake timer for windows updates. 

Thank you very much.

I have disabled that setting. I have upgraded the BIOS to the latest version which according to Dell is v2.1.2 and it still turns on by itself. I read a thread on Reddit that this is a quirk/bug in the Ryzen BIOS. Some PC's do it and others don't. I also read on Reddit that if you have this problem it will turn itself on at the time you flashed the BIOS. I can't confirm the above as to why some R10's just power on automatically.

I've just gotten to the point that I unplug my R10 at night because if I don't it will continue to run and not go into sleep mode because to get to the desktop I have to enter my password. So the PC never gets a chance to enter sleep mode. Other than that everything else works.

I had this problem, this is the fix:

-Enter bios and enable the setting that wakes up the computer, and pick a day.

- Save, reboot and enter windows.

- Reboot, enter bios again and disable the setting.

It will now stop turning itself on each day.

I have this exact issue and it goes away by doing what Vanadiel just stated above. No need to keep physically pulling the plug :). It does come back after each bios flash so just need to keep repeating the mentioned process once after each flash. Annoying but manageable.